In the section on Predicate Proofs in chapter 11, this document states:
"Using zero-knowledge methods, predicate proofs can be generated by the holder at the time of presentation without issuer involvement. For example, a verifiable credential with the claim birthdate can be used in a verifiable presentation to prove age-over-18. The same credential could then be used in another presentation to prove age-over-25, all without revealing the holder's birthdate."
I understand that a zero knowledge proving system allows a holder to demonstrate that she knows a claim (which is authenticated by an issuer) and that she used that claim inside a defined and known computation to derive some other value. She will moreover be able to prove to a verifier that the new value was indeed correctly derived from the claim, without revealing the claim itself.
However, I think that this does not justify the given example of proving age-over-25 based on proven knowledge of birthdate. The problem I see that for this particular calculation, the holder needs to use the current date (or at least the date on which the age-over-25 claim is to have value TRUE). That current date cannot be included in the claim that she received from the issuer, because we assume no involvement of the issuer. Therefore, the current date must originate from the holder itself. But that means that in order to accept the age-over-25 claim, the verifier must trust the holder. And this obviously defeats the purpose of the claim.
Perhaps my thinking above is wrong. In that case, I think that a clarification would be helpful also for other readers. Please note that the same example is used at other places as well, for example section 12.3. Figure 11 in the VC Data Model suggests the same example.
If I am right, however, we need to find another example to demonstrate the power of ZKPs for real-life applications of VCs.
In the section on Predicate Proofs in chapter 11, this document states:
"Using zero-knowledge methods, predicate proofs can be generated by the holder at the time of presentation without issuer involvement. For example, a verifiable credential with the claim birthdate can be used in a verifiable presentation to prove age-over-18. The same credential could then be used in another presentation to prove age-over-25, all without revealing the holder's birthdate."
I understand that a zero knowledge proving system allows a holder to demonstrate that she knows a claim (which is authenticated by an issuer) and that she used that claim inside a defined and known computation to derive some other value. She will moreover be able to prove to a verifier that the new value was indeed correctly derived from the claim, without revealing the claim itself.
However, I think that this does not justify the given example of proving age-over-25 based on proven knowledge of birthdate. The problem I see that for this particular calculation, the holder needs to use the current date (or at least the date on which the age-over-25 claim is to have value TRUE). That current date cannot be included in the claim that she received from the issuer, because we assume no involvement of the issuer. Therefore, the current date must originate from the holder itself. But that means that in order to accept the age-over-25 claim, the verifier must trust the holder. And this obviously defeats the purpose of the claim.
Perhaps my thinking above is wrong. In that case, I think that a clarification would be helpful also for other readers. Please note that the same example is used at other places as well, for example section 12.3. Figure 11 in the VC Data Model suggests the same example.
If I am right, however, we need to find another example to demonstrate the power of ZKPs for real-life applications of VCs.